My (basic) understanding of lenses is that their refraction is largely determined by the material (index of refraction) and shape.
It seems possible to make lenses out of deformable materials, which I assume you can change the shape/curvature to change the lens behavior.
My question is are there lens materials that changing their stress/strain/density/mechanical deformation without changing their outer shape can change lens behavior?
Apologies if this is badly worded, and I'm open to suggestions for better framings. I'm pretty new to this.